Shabbat 47 - Ash
Rabbi Assi stated that Rabbi Yehudah the Prince permitted moving a censer on Shabbat, along with its ashes. Rabbi Zeira said to him, "How could this be? Granted that one may move a basket with a stone inside, but that is only because this basket also has fruit in it; censer has only ashes, and they are not fit for use on Shabbat, and therefore muktzeh!"
Rabbi Assi was confounded for a moment, and then said, "Here too we are dealing with a censer that has granules of unburned incense! They are not muktzeh, and the censer can be carried because of them." However, Abaye rejected this explanation: "Granules are not considered as anything in the household of Rabbi Yehudah the Prince. Rather, Rabbi Yehudah wanted the unpleasant censer moved away." Rava, in turn, rejected this: "Censer is not repugnant, and besides, the ashes in a censer are covered. Rather, the ashes themselves were planned to be used on Shabbat, for covering dirt, and that is why they were not muktzeh, in fact, they were the important part, even if the rest was muktzeh."
We thus see the rule: a muktzeh object may be moved together with another, permitted object, if they are together in one container, and if the permitted object is the more important one.
Going back to the Shabbat lamp, one may put a vessel under it, to catch the falling sparks, but not place water in that vessel. How is this different from a plate to collect dripping oil, which was forbidden? - Sparks are insignificant. Then why not put water? - Because that would be too close to actively extinguishing them.
Art: Wolfgang Heimbach - Young woman with an oil lamp